feat: use cobra to provide subcommands, move sources to lib (#506)

- Use cobra in order to provide subcommands `serve` and `db`.
  - Subdir `cmd` is removed.
  - Subdir `cli` is created, which is a standard cobra structure.
- Sources related to the core are moved to subdir `lib`.
- #497 and #504 are merged.
- Deprecated flags are added. See https://github.com/filebrowser/filebrowser/pull/497#discussion_r209428120.
- [`viper.BindPFlags`](https://godoc.org/github.com/spf13/viper#BindPFlags) is used in order to reduce the verbosity in `serve.go`.


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Former-commit-id: 07bc58ab2e1ab10c30be8d0a5e760288bfc4d4dc

cli/cmd/rootalias.go

@@ -0,0 +1,46 @@
+package cmd
+
+import (
+	"log"
+	"os"
+)
+
+// checkRootAlias compares the first argument provided in the CLI with a list of
+// subcmds and aliases. If no match is found, the first alias of rootCmd is added.
+func checkRootAlias() {
+	l := len(rootCmd.Aliases)
+	if l == 0 {
+		return
+	}
+	if l > 1 {
+		log.Printf("rootCmd.Aliases should contain a single string. '%s' is used.\n", rootCmd.Aliases[0])
+	}
+	if len(os.Args) > 1 {
+		for _, v := range append(nonRootSubCmds(), []string{"--help", "--version"}...) {
+			if os.Args[1] == v {
+				return
+			}
+		}
+	}
+	os.Args = append([]string{os.Args[0], rootCmd.Aliases[0]}, os.Args[1:]...)
+}
+
+// nonRootSubCmds traverses the list of subcommands of rootCmd and returns a string
+// slice containing the names and aliases of all the subcmds, except the one defined
+// in the Aliases field of rootCmd.
+func nonRootSubCmds() (l []string) {
+	for _, c := range rootCmd.Commands() {
+		isAlias := false
+		for _, a := range append(c.Aliases, c.Name()) {
+			if a == rootCmd.Aliases[0] {
+				isAlias = true
+				break
+			}
+		}
+		if !isAlias {
+			l = append(l, c.Name())
+			l = append(l, c.Aliases...)
+		}
+	}
+	return
+}